Interlock systems that control the operation of various types of machinery are well known. In most instances, these interlock systems are required to be hardwired, electromechanical, and self-checking. The interlocks typically consist of a number of normally closed contacts connected in series to energize a control relay. These interlocks include, but are not limited to emergency stop push buttons, limit switches, open door indicators, palm switches, and so on. As long as the interlocks are closed, the relay remains energized and the machine can operate. Opening any one of the interlocks causes the relay to deenergize and the machine is shut down. These normally closed contacts will always have a finite voltage drop across them. With more complex machinery, the number of contacts wired in series becomes very large. The ohmic losses across these contacts is such that the sum of these voltage drops may prevent the control relay from energizing. As a result, other systems must be utilized. One method to overcome this drawback is to divide the interlocks into smaller groups of series interlock connections and then use each one of these groups to energize a separate interposing relay. The contacts from these separate interposing relays are then connected in another series connection to energize the final control relay that controls the operation of the machine.
Whereas this method may provide sufficient control in some simple applications, other interlock systems require redundant controls and the ability to provide a means for self-checking the contacts to determine if they open and close properly. The present invention provides a self checking control system that addresses these and other problems.